I would suggest making the back bends to the grill as you planed but using the side/angled edges to attach the grill to the horn edge.
This is what I did in the case of the (late...) Omni 10.5 some years ago. the angled horn sides practically lend themselves for this solution...
I chose 3 holes per side on the bent grill edges, that where in line with holes on the front of the grill so that screws could be nicely accessed with a screwdriver from the front,
then used 1/8" thick rubber washers both between the grill edges and the horn side, and also under the wide washer under the screw head. The rubber washers snugly fit the screws so nothing fell while installing the grill.
By this method the attaching screws are nicely hidden while supplying a good hold for the grill.
I wouldn't recommend leaving any free edge (without a bent rib, or a fastening screw) longer than a few inches, this will have very poor resistance against even mild abuse and also may vibrate.
Also make sure the grill edge is ether in the air or pressed to the cab with some weather stripe or other cushioning material.
